In general, surface plasmon refers to quantized collective vibration of charge density on a metal surface. The surface plasmon is combined with various electronic waves, such as visible rays and infrared rays provided under specific conditions, and is allowed to propagate along the boundary surface between a metal and a dielectric substance, thereby forming surface plasmon polariton.
The surface plasmon generated in metal nanoparticles with a nanometer size is confined in the metal nanoparticles and vibrates therein. Herein, the resultant surface plasmon polariton is referred to as localized surface plasmon polariton.
The above-mentioned plasmon phenomenon has been used to realize a nanophotonics device below the diffraction limit, or to develop a light emitting diode (LED), solar cell and a sensor using a strong effect of enhancing electromagnetic field.
Meanwhile, an up-conversion substance is one capable of converting electromagnetic waves with a long wavelength into those with a short wavelength. Basically, such an up-conversion substance is a material absorbing two or more low-energy photons to emit one high-energy photon.
Studies about such an up-conversion substance are slightly insufficient as compared to the quantum dot or a down-shifting substance. However, according to the reports up to date, NaYF4 doped with both ytterbium (Yb) and erbium (Er) is being developed. Such NaYF4 includes the most efficient particles characterized in that they convert near-infrared rays into visible rays. However, in this case, the photoluminescence efficiency of up-conversion is less than 3%, which is significantly low. Thus, there is a problem related with commercialization (Non-patent Document 1).
Therefore, many studies have been conducted to date to solve the above-mentioned problem. However, according to the related art, there is no increase in photoluminescence intensity (Non-patent Document 2), or the photoluminescence intensity merely increases by 3.8 times up to at most 310 times (Non-patent Documents 3 and 4).
Thus, the up-conversion substances according to the related art have insufficient photoluminescence intensity, and have difficulty in commercialization because they are produced through complicated processes and conditions.
Under these circumstances, the present inventors have conducted intensive studies. The inventors have found that when up-conversion nanoparticles are inserted into a metal-insulator-metal structure, gap plasmon formed by a combination of localized surface plasmon with surface plasmon polariton may be excited to maximize the wavelength converting efficiency and photoluminescence intensity. The present invention is based on this finding.